The first steps in understanding the cochlea, the mammalian hearing organ, were achieved by H. L. F. Helmholtz, who revealed in 1863 the tonotopic principle, followed by von Békésy's discovery of traveling waves along the basilar membrane (BM), and Gold's conjecture of active amplification in the cochlea (1948) which was evidenced by the discovery of otoacoustic emissions (the production of sounds by the ear itself). Since then, various experiments revealed that the locus of active amplification is in the outer hair cells (OHC), that reside on top of the basilar membrane.
Recently, several authors have suggested that Hopf-type instabilities may be responsible for some of the observed features of the biological cochlea (V. M. Eguiluz et al., Phys. Rev. Lett. 84, 5232 (2000); M. O. Magnasco, Phys. Rev. Lett. 90, 058101 (2003); A. Kern and R. Stoop, Phys. Rev. Lett. 91, 128101 (2003); R. Stoop and A. Kern, Phys. Rev. Lett. 93, 268103 (2004); R. Stoop et al., Physica A 351, 175 (2005)). In these papers, some features of the biological cochlea have been reproduced by solving, for stationary input signals, differential equations which correspond to specific mathematical models of the cochlea.
For transient signals, however, computational effort would be too high in order for the suggested methods to be useful in practical applications. No electronic implementations of a cochlea model have been suggested by these authors.
A realistic electronic model of the cochlea is highly desirable in the development of advanced hearing aids such as cochlear implants and in technologies such as robotics where an analysis of the auditory scene is often required. In the past, several attempts have been made to devise an electronic cochlea mimicking the physiological response characteristics of the biological cochlea, in particular the amplitude or velocity distribution along the basilar membrane. Several models have been proposed which use a bank of band-pass filters, all filters receiving the same input signal and each filter being coupled to an amplifier (filter bank models). An early example is U.S. Pat. No. 4,063,048. Other examples include U.S. Pat. Nos. 5,402,493, 5,848,171, 5,500,902 and 6,868,163. Other models are based on a transmission line design, as proposed, e.g., in U.S. Pat. No. 5,030,198. However, often the responses measured on such implementations significantly differ from physiological measurements. In addition, such implementations tend to be rather complicated.